This is what is called a false narrative.
The US involvement is S. Korea had its roots in the geo-strategy of securing western liberal democracy and capitalism from Communism.

Korea is one of the last places where the original circumstances and hence the motivation still prevail.
Japan is a strategic US ally. If you are worried about China then you are also worried about North Korea. And the consequences of Japan (or Korea) being an ally or subject of China should be pretty obvious.

Secondarily, I’m not sure if you care about a world order that is predicated on western liberal democracy and capitalism being worth defending. But if you do, then you must be a little confused about your argument. Or misinformed.

Apart from China being the principle beneficiary of your narrative, but Russia is an open advocate for deconstructing this western liberal capitalistic enterprise. And it is also a central source for propagating your narrative. For obvious reasons. If you’re uncertain about those reasons, let me know. I can point you to plenty of resources you can read and check.

Just a final couple notes, since I take it you’re not too familiar with South Korea. First of all, it has universal (male) military conscription. It spends 2.6 percent of its GDP on the military. However, this is calculated differently from the USA’s 3.2%. It does not take into account the loss of productive labor from its entire male population for an average of about 1year7months. Also, Korea foots the bill for all the infrastructure for both its military and ours. If you watched “Parasite”, this year’s winner of best movie which is Korean, you get an idea of how difficult life is there. One of the reasons that so many Koreans emigrate to the USA. And, of course, typically achieve the highest levels of professional success of any immigrant group. Mostly business owners, scientists, engineers and medical professionals.

So please think long and hard about your inaccurate understanding of this issue.